Electrochemical method for coating steel surfaces and the product thereof



Nov. 5, 1957 R. A. NEISH 2,812,296 ELECTROCHEMICAL METHOD FOR comma STEEL SURFACES AND THE PRODUCT THEREOF Filed June 22, 1956 FIc5. Z

TEMPER-ROLLED LOW- CARBON STEEL STRIP .010'- .OI8'THIGK WATER SOLUTION CONTAINING a-mav vrr. OF NQ Gr fl-fZH- O a 2% BY VOL. 0F 85% H 90 HEATED T0 ABOUT IZO'F. SI'RlP CONNECTED AS OATHODE. cunnsm'ozusmsomps. asm.

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SHE-ETS SPRAYED ,wma LAOOUER INVENTOR RICHARD A. NEISIH BY Amaa HIS ATTORNEYS United States PatentO ELECTROCHEMICAL METHOD FOR COATING STEEL SURFACES AND THE PRODUCT THEREOF Richard A. Neish, Snowden Township, Allegheny County,

Pa., assignor to United States Steel Corporation, a corporation of New Jersey Application June 22, 1956, Serial No. 593,217

9 Claims. (Cl. 204-48) This invention relates to a process for treating the sur faces of steel products such as ordinary uncoated sheet steel (black plate) to increase its corrosion resistance and the adherence of the lacquer coating usually applied to can stock, so it may be used safely and satisfactorily as a substitute for tin-plate in the manufacture of cans intended for packing certain commodities which have heretofore required tin cans.

This is a continuation-in-part of my application Serial No. 338,456, filed February 24, 1953, for Electrochemical Method for Coating Steel Surfaces and the Product Thereof, now abandoned, which was, in turn, a continuation-in-part of my application Serial No. 203,627, filed December 30, 1950, for Method of Treating Sheet Steel to Adapt it for Use as Container Stock and Product Thereof, now abandoned.

Containers made from lacquered black plate have been used for packing dryproducts and non-corrosive products 'such' as lubricating oil, The use of lacquered black plate containers for the packaging of foods and, certain other 'wet products, however, has not been feasible. The ad- :herence of .the lacquer to the steel is often insufiicient to known commercial phosphating process includes precleaning, rinsing, phosphating, rinsing,-dichromate rinsing, and drying. The.solutions are carefully balanced mixtures that must be. properly controlled, and several sets of variable-speed scrubbing rolls are used as accessories.

The use of phosphate coatings on steel to improve the minimum time of 10 seconds is required for the above steps, and steel manufacturers find such a process cumbersome and expensive. Many other processes are known and have been attempted commercially, but comparatively .few are in use .today because of defects inherent to change the type of lacquer. Furthermore, significantly I heavier lacquer coatings cannot be used because fabrication of the container causessuch heavier coatings to flake oil the deformed, areas. When the lacquer coatings are held within the customary. ranges of thickness, the lac- "quers usually'lose theiradherence, particularly in the headspace end of the can, during the processing of food products. This has been the major reason for the ultimate failure of what might otherwise have been successful tinless containers for many of the comparatively noncorrosive food products.

I have invented a novel method of processing sheet steel such as that used as container stock, to improve the adherence of lacquer coatings thereto and retard corrosion of the metal under the coating, which is simple, rapid, economical and easy to practice and control. I have found that, when uncoated sheet steel, for example, is made the cathode in an acidic solution containing both phosphate ions and the ions of chromium salts in which the chromium is hexavalent, within certain ranges of concentrations and within a certain temperature range, and a thin film of the solution is spread evenly over the metal surface and dried, I obtain a thin, tightly adherent film thereon which is very suitable as a base for lacquer. This film can be produced by my method at a rate of speed and a degree of economy and commercial simplicity previously unknown.

drawings:

Figure 1 is a diagrammatic showing of the apparatus .for performing my method; and

Figure 2 is a similar view showing the lacquer coating of the treated sheets.

Referring in detail to the drawings, I uncoil steel strip .10 having a low carbon content, i.e., .03 to .15%, which -.has been cold-rolled to the desired finished gage, from a coil-holding reel 11 and pass it in single ply through a processing tank 12. The tank is fitted with guide rollers 13 and contains a bath of treating solution 15, the rollers being disposed so as to cause the strip to be progressively immersed in the bath during its travel into and out of the tank. Squeegee or wringer rolls 16 at the exit end of the tank wipe excess solution from the strip leaving it uniformly covered with a thin liquid film.

Bath 15 is an acid aqueous solution containing phosphate ions and the ions of chromium compounds in which the chromium is hexavalent, and has a pH value from about 1 to 2. The phosphate ions may be supplied by I phosphoric acid or by a soluble phosphate such as sodium,

potassium or ammonium phosphate, and the hexavalent chromium by chromic acid or any soluble dichromate. If neither phosphoric nor chromic acid is used, the desired acidity may be obtained by the addition of nitric acid. The solution may conveniently be prepared by adding to .water from 1 to 6% sodium dichromate by weight and from .75 to 6% by volume of ortho-phosphoric acid This gives a solution containing from 1.0 to 8.4% of the phosphate radical and from .3 to 2.1% chromium, both by weight. A preferred composition is:

2% Na2CrO'7-2H2O by weight 2% HsPO4 (85%) by volume and the balance substantially water.

This solution has a pH value of about 1. The solution may also be prepared by using chemically equivalent amounts of other salts or acids as stated above.

Although the essential composition of my treating solution is as above described, I may find it desirable to add other salts to the solution to obtain further improvement in the character of the coatings. For example, I may add from 1 to 4% of sodium nitrate (preferably 1%), or 0.1 to 0.4% of potassium permanganate (preferably 0.2%). ;These help to produce a uniform coating of the desired -type, and "tend to improve the properties of the film.

Although either can be used, the nitrate is the more stable under commercial operating conditions. If NaNOa is used, the total NaNOa including that from the nitric acid for control of pH, should not exceed the equivalent of 4% NaNOs.

The bath 15 serves as the electrolytein a cell-connected to a current source such as a direct-current generator 17. The strip passing through the tank is made the cathode in this circuit. The negative terminal of the generator, for example, may be connected by conductors 18 to guide rollers 13 at the entrance and exit sides of tank 12. The other terminal of the generator'is connected by conductors 19 to anodes 20 of stainless steel or other suitable material immersed in the bath 15. 'The voltage of the generator is controlled to cause current flow between the anodes and the strip at a current density depending somewhat on the speed of travel of the strip through the tank.

For the purpose of the treatment of my invention, it is desirable that the strip be immersed-in the bath fora time of from .25 to 2 seconds, preferably about '1 second, and the total length of the anodes is made such thatthe average speed of the strip will produce this result. At the minimum time, the current density should "be about 90 ampsnp. s. f. and at the maximum, about "15 amps. p. s. f. For the-preferred'treatingtime, the current density should be from about 30'to 601111113891). s. f. Atreatment at '60 amps. p. s. 'f. prolonged to 30 seconds does not produce aheavier or more effective coating.

For most efficient operation, the bath 15 is maintained at a temperature between 100 and 160 F. and :preferablybetween 120 and 140 F.,'by any convenient means such as steam coils immersed therein. At lower 'bath temperatures, the film applied to the strip, after drying, is heavier but is chalky providing neither adequate corrosion resistance nor lacquer adherence. At higher temperatures, the film is invisible and the lacquer adherence is impaired. Films produced at a bath temperature of from about 120 to 130 F. are the most effective in improving the corrosion resistance and lacquer adherence and are smoky gray in appearance.

The cathodic electrolytic action to which the strip is subjected reduces the metallic oxide existing on the sur face of the steel and forms a thin boundary film of a microcrystalline phase of unknown composition. It is believed that this microcrystalline phase then partially reacts with the solution film that is dragged out on the strip; the remainder of the solution film tending to seal the coating when it is dried. The exact nature of the reaction or the character of the coating produced on drying the residual film of the solution is not known precisely but the effect thereof in improving corrosion resistance and lacquer adherence is readily determinable @by simple tests.

The available evidence shows that a complete film made according to the described procedure consists of (a') a microcrystalline phase which contains phosphate and is directly adjacent and tightly adherent to the metal surface, and (b) an amorphous gel-like phase, which contains both phosphate and chromate and is adsorbed in and on the surface of the mircocrystalline phase. Chemical analysis of the film on specimens of semi-commercial product and on laboratory specimens has shown that both phosphorous and chromium are present. In this analysis, the actual chemical forms of the above elements were not identified. A study of the complete film by reflection electron diffraction disclosed that the outer surface of the film was amorphous.

In additional tests, specimens were submitted to the electrolytic filming treatment and then immediately and thoroughly rinsed with water to remove adsorbed treating solution before the films were dried. This Waterrinsing technique apparently prevented the formation of the outer amorphous phase, and electron-diffraction examination was then utilized to confirm *the presence of a microcrystalline phase. The outer amorphous phase is believed to be a complex gel apparently formed from adsorbed phosphate-chromate solution and primary reaction products that may be present. Microchemical analysis of the crystalline phase has proven that phosphate is present in this phase although the test for chromate ion was negative. The exact chemical or structural form of the crystalline phase has not been identified. Other experiments have shown that the microcrystalline phase does not form when the treating solution contains only phosphoric acid. Therefore, although its exact function is not understood, the chromate addition is essential.

Coatings produced by simple immersion in the treating bath, that is, without electrolytic action, were markedly different by visual inspection from those produced by the electrochemical treatment. There was little visible coating on sheet iron after as much as two minutes immersion, in contrast to the readily apparent coatingproduced by the electrochemical treatment within two seconds. Electron-diffraction analysis of the immersion film, after rinsing to make any crystalline phase discernible, revealed only a sharp pattern of gamma FeO (OH), which is ,a common form of rust. No phosphate or chromate was detected in the above immersion coating by microchemical analysis.

A satisfactory protective film cannot be applied without the electrolytic action. When it 'is attempted, the resulting film is spotty and non-uniform giving incomplete coverage and has a tendency to smear in a steamy atmosphere, and the adherence of lacquer to this coating is not satisfactory. The solution can be varied appreciably without affecting the fundamentals of the process, provided that both phosphate and hexavalent chromium are present. Chromic acid or dichromate alone does not produce the desired film. Acidic phosphate solutions without 'hexavalent chromium will produce films, but'these films are definitely inferior.

With prior p'hosphating methods, the solutions contain appreciable quantities of zinc .or manganese phosphates. .Such additions are said to be essential in order toprevent the lattice structure of the coating from changing when exposed to corrosive conditions. My coatings have suitable corrosion resistance, even though zinc and manganese phosphates are absent.

With prior phosphating solutions and methods, furthermore, strong oxidizing agents, such as sodium dichromate cannot be used in concentrations greater than traces. The reasons are that (1') the sodium dichromate oxidizes ferrous ions to ferricions during the phosphatingof steel, so that the balance and the functioning of the solutions are disturbed, and (2) the sodium dichromate inhibits the action of phosphoric acid and tends to passivate the steel surface, so that the desired coating is not formed. This isin direct contrast to my treating solution, which contains appreciable quantites of sodium dichromate.

The squeegee rollers :16 are preferably provided with means such as screws for adjusting the pressure exerted thereby and thus controlling the amount of the liquid film left on the strip to dry. This residual film should be the minimum which, when dried, will be suflicient to give the smoky gray underlayer the maximum corrosion resistance and lacquer adherence. I .find .that a dried film weighing from 6 to 30 milligrams p. s. f. of surface is most effective. The film may be dried by natural evaporation if time and space limitations permit. I prefer, however, to dry the film by passing the strip through a hot-air drier 21 having an air inlet',22 and an outlet 23, or equivalent drying means. The drying is facilitated by the fact that the strip, on emerging from the bath, is at the temperature of the latter and tends, therefore, to lose moisture rapidly to the surroundingair. Beyond the drier 21 is a drive bridle 24 which pulls the strip through the processing line. The drive bridle delivers the strip to aflying shear 25 which cuts itinto sheets. The severed sheetsfall onto a piler26.

It will be appreciated from the foregoing that the sheets reaching the piler have a dried film or coating thereon which has been produced according to the techings disclosed herein.

The films applied by my invention, when 'dry, are very thin and tightly adherent and have a grayish coloration. They are visible on the metal surface as the strip leaves the electrolytic bath, are already an integral part of the surface and are not removed or destroyed by normal handling even before they are dry. They thus differ from anodic or immersion films produced by baths similar to the described bath, which are like paint films and are very easily damaged while they are still wet. These immersion films consist almost totally of semi-soluble gelatinous compounds.

It appears that a thin, tightly adherent dense film of metallic phosphates is formed and that this absorbs and holds hexavalent chrominum (presumably in the form of a semi-soluble heteropolar gelatinous complex compound) within the structure and on the surface of the film of metallic phosphates.

Coatings made according to the described process, with or without either of the above addition agents in the phosphate-chromate solution, are suitable for lacquering, enameling, or painting as shown in Figure 2. The coatings can be further improved, however, by a secondary treatment with a suitable solution.

Examples of two complete procedures are as follows:

Example 1 (a) Steel is cathodically treatedaccording to the following:

Solution:

2% H3PO4 by volume 2% Na2Cr2O'z-2H2O by weight 0.2% KM1104 by weight Temperature-120 F. Current density60 amps. p. s. f. Treating time-1 second or more (b) The treated steel is squeegeed, dried, rinsed with the above treating solution, squeegeed, and dried again.

Example 2 (a) Steel is cathodically treated according to the following:

Solution:

2% H3PO4 by volume 2% NazCrzOq-ZHzO by weight 1% NaNOa by weight Temperaturel20 F. Current density40 amps. p. s. f. Treating time-1 second or more (b) The treated steel is squeegeed, dried, rinsed with a water solution containing 2 /2 chromic acid, squeegeed, and dried again.

Filmed steel strip produced according to the above method has been tested in many different ways. The resistance of individual sheets to atmospheric humidity is not equal to that of tin plate, but when the treated steel is piled and wrapped in the customary fashion, its storage life is quite suitable for the purpose. On the other hand, this treated steel has proved resistant in steam tests, in water-immersion tests, and in salt-spray tests. The resistance shown in these latter tests is greatly extended by lacquering. Cans made from treated and lacquered strip have been subjected to food-pack tests, and the results have been satisfactory. It has been found that treated and lacquered strip may be drawn or formed, within the limits of the distensibility of the lacquer being employed, without flaking or peeling of the lacquer. The lacquer adherence has also been tested by forming and doubleseaming can ends, and by the use of cup machines and bend tests. On the basis of the above tests, it is believed that a tinless container may be formed from lacquered steel strip that has been treated according to the described process. Furthermore, such tinless containers may be used for packing or canning a variety of products which cannot be packed satisfactorily in untreated lacquered steel.

My improved filming process is particularly intended for the rapid, continuous treatment of cold-reduced steel strip in coils, but it is just as suitable for the treatment of sheet, pipe or formed products. The filming process can beoperated at a rate of speed and with a degree of simplicity and economy previously unknown, and it is thus a marked improvement over any existing method, from the standpoint of production rate or throughput.

I am aware that films somewhat similar in appearance can be produced by cathodic electrolytic treatment in baths containing acid phosphate alone, nevertheless the presence of hexavalent chromium in the solution is essential if maximum corrosion resistance is to be imparted to the films.

The films produced by my method may be considered as extremely thin barrier films which retard the passage of moisture, yet are ideally suited as a base for lacquers, etc. According to my estimations, the underfilm usually weighs less than .12 milligrams p. s. f. of surface area, andthe total film weight is usually less than 50 milligrams p. s. f. The film made by the disclosed process is thus thinner'than the usual phosphate films. The thinness of the coating is beneficial because thin films are usually more ductile for drawing and forming operations and usuallycontain less semi-soluble components. When the coating process is performed as explained herein and the squeegee rolls are properly adjusted, it is practically impossible to obtain films that are too heavy.

' The product of the method described is resistant to corrosion when tested in water, steam and salt spray, and this resistance is greatly increased by a coating of lacquer such as that ordinarily applied to tin plate to be used as can stock, as shown in Figure 2. Such coating is thus much less likely to fail through underfilm corrosion than a similar coating applied to untreated steel sheets. The lacquer adherence is also materially improved by the treatment. Flaking and peeling of the lacquer coating does not occur on normal drawing or forming of the sheets. The product is thus adapted for use as a substitute for tin plate for packing many products for which untreated lacquered sheets are unsuitable. The surface film formed by this process also takes and holds paint well.

The method described is characterized by numerous advantages. It is simple, rapid and easy to control. Strip-processing speeds of several hundred feet per minute are easily obtainable. Scrubbers, cleaning solutions and rinses to remove cleaning solutions are not normally required as a pretreatment to obtain satisfactory coatings. No pretreatment nor after-treatment such as cleaning or rinsing is required although a final rinse is desirable.

By a slight modification of the process described above, I am able to turn out a product having a corrosion resistance even greater than that of the product of the described process, and a lacquer adherence equally good. The modification of the process involves only the introduction of an anodic electrolytic treatment, similar to the cathodic treatment and subsequent thereto, in the same or a similar electrolyte. In respect to apparatus, this may be effected by the provision of a duplicate of tank 12 and its contents and a second generator 17, the negative terminal of which is connected to the electrodes 20 of the Second tank, all other conditions being generally the same in the two tanks, i. e., time, temperature, pH and current density.

I have found that the electrolytic treatment of the strip as anode, subsequent to the treatment as cathode as already described, not only does not impair the corrosion resistance of the product before lacquering or the lacquer adherence of the product after lacquering, as might be expected, but materially increases the corrosion resistance oi the; unlacquered. product. The. product; also: has. adifferent appearance,. i.. .e;,. arbrownislncoloration.v 1 Although. I: have disclosed, herein: a preferred practice of my invention and amodification, I intendzto. cover.- as well any other change or modification therein which mayv be; made. without. departing frormthe spirit. and scope of the invention. 4

Iclaim: a

1. In amethod. of coating sheet steel, the.-steps.includ* ing passing it. through .a, solution, consisting essentially" of from 1. to 6% of a water-soluble chromate in which the chromium is hexavalent; water and a. compound contain-v ing. the orthophosphate. radical, the amount ofsaida compoundv being such as. to..pr.ovide. an. orthophosphate-ion concentration in. the solutionaof-irom L to 8.4%, said solution having a pH valueof fromabout 1 to 2, subjecting the. steel to electrolysis as cathode while in the solution, at a current density. of from 15. to. 90' amps. p. s. f., then removing the steel from the solution and at least partially drying. the liquid. film adhering thereto.

2. The. method defined by claim .1 characterized by said solution containing also from 1 to 4% of sodium nitrate.

3. The method defined by claim 1 characterized by said" solutioncontaining also. from .1 to.4% potassium permanganate.

4'. The method defined by claim 1 characterized by maintaining said solution at a temperaturebetween 100 and. 160 F. a

5. The method defined by claim 1 characterized by continuing said electrolysis fora period of from .25 to 2seconds. a I

6. The method. defined by claimi 11. characterizedby rinsingthe coated; dried; steel withzsaidzsolutiomt. ZaJ'Ehe method defined. b.y-:claim.-f1 characterizedbyi rinsing the coated, dried steel with a dilutelwater'solutioni ofichromiccacid... z: Ii t? 8.. The. method. as defined: by claim lg'characterizeds by subjecting the; strip; after. the; cathodic .at'reiatment, to a second electrolytictreatmentaas anodein an-electrolyte, andunder conditions, similar-tothoseused; in the cathodic. treatment. 1 x .1. j

9. As an article; of manufacture; a metal articlehaving a. steel surface with a coating thereon; formed by passing the article througlra solutiom consisting. essentially ofv from 1. to 6%& ofawater-soluble.chromatei-in which the chro-i mium is hexavalent, water and a compound containing; the orthophosphate radical; thelamojunt of said; compound being, such as, to provide ancorthophosphatedm concen.-: tration inv the solution. of from. 1 .010: 8.4%., 821151 501111.1011 having apH value of fromv about 11' to. 2,; subjecting thesteel to electrolysis as cathode while in the solution, at a current density of from 15 to amps. p. s. f.', then: removing. the steel from the solution and at least partially dryingitheliquidzfilm adheringthereto.

References Cited'in thefile of this patent UNITED. STATES PATENTS 1,007,069 Coslett Oct. 31, 1911 2 ,132,439; Romig- Oct. 11, 1938 2,303,242 Tanner Nov. 24, 1942 2,590,927 Brandt et al. Apr. 1, 1952 2,606,866 Neish Aug. 12, 1952 2,769,774 Loveland et al Nov. 6, 1956 

1. IN A METHOD OF COATING SHEET STEEL, THE STEPS INCLUDING PASSING IT THROUGH A SOLUTION CONSISTING ESSENTIALLY OF FROM 1 TO 6% OF WATER-SOLUBEL CHROMATE IN WHICH THE CHROMIUM IS HEXAVALENT, WATER AND A COMPOUND CONTAINING THE ORTHOSPHOSPHATE RADICAL, THE AMOUNT OF SAID COMPOUND BEING SUCH AS TO PROVIDE AN ORTHOPHOSPHATE-IONCONCENTRATION IN THE SOLUTION OF FROM 1.0 TO 8.4%, SAID SOLUTION HAVING A PH VALUE OF FROM ABOUT 1 TO 2,SUBJECTING THE STEEL TO ELECTROLYSIS AS CATHODE WHILE IN THE SOLUTION, AT A CURRENT DENSITY OF FROM 15 TO 90 AMPS. P.S.F., THEN REMOVING THE STEEL FROM THE SOLUTION AND AT LEAST PARTIALLY DRYING THE LIQUID FILM ADHERING THERETO.
 9. AS AN ARTICLE OF MANUFACTURE, A METAL ARTICLE HAVING A STEEL SURFACE WITH A COATING THEREON FORMED BY PASSING THE ARTICLE THROUGH A SOLUTION CONSISTING ESSENTIALLY OF FROM 1 TO 6% OF A WATER-SOLUBLE CHROMATE IN WHICH THE CHROMIUM IS HEXAVALENT, WATER AND A COMPOUND CONTAINING THE ORTHOPHOSPHATE RADICAL, THE AMOUNT OF SAID COMPOUND BEING SUCH AS TO PROVIDE AN ORTHOPHOSPHATE-ION CONCENTRATION IN THE SOLUTION OF FROM 1.0 TO 8.4%, SAID SOLUTION HAVING A PH VALUE OF FROM ABOUT 1 TO 2, SUBJECTING THE STEEL TO ELECTROLYSIS AS CATHODE WHILE IN THE SOLUTION, AT A CURRENT DENSITY OF FROM 15 TO 90AMPS. P.S.F., THEN REMOVING THE STEEL FROM THE SOLUTION AND AT LEAST PARTIALLY DRYING THE LIQUID FILM ADHERING THERETO. 